In designing an integrated circuit (IC), engineers typically rely upon computer simulation tools to help create a circuit schematic design consisting of individual devices coupled together to perform a certain function. To actually fabricate this circuit in a semiconductor substrate the circuit must be translated into a physical representation, or layout, which itself can then be transferred onto the silicon surface. Again, computer aided design (CAD) tools assist layout designers in the task of translating the discrete circuit elements into shapes which will embody the devices themselves in the completed IC. These shapes make up the individual components of the circuit, such as gate electrodes, field oxidation regions, diffusion regions, metal interconnections, and so on. The software programs employed by these CAD systems are usually structured to function under a set of predetermined design rules in order to produce a functional circuit. Often, these rules are determined by certain processing and design limitations. For example, design rules defining the space tolerance between devices or interconnect lines so as to ensure that the devices or lines do not interact with one another in any unwanted manner.
Design rule limitations are frequently referred to as critical dimensions. A critical dimension of a circuit is commonly defined as the smallest width of a line or the smallest space between two lines. Consequently, the critical dimension determines the overall size and density of the IC. In present IC technology, the smallest critical dimension for state-of-the-art circuits is 0.5 microns for line widths and spacings.
Once the layout of the circuit has been created, the next step to manufacturing the integrated circuit (IC) is to transfer the layout onto a semiconductor substrate. Photolithography is a well known process for transferring geometric shapes present on a mask onto the surface of a silicon wafer. In the field of IC lithographic processing a photosensitive polymer film called photoresist is normally applied to a silicon substrate wafer and then allowed to dry. An exposure tool is utilized to expose the wafer with the proper geometrical patterns through a mask by means of a source of light or radiation. After exposure, the wafer is treated to develop the mask images transferred to the photosensitive material. These masking patterns are then used to create the device features of the circuit.
Commonly, when transferring a pattern to a semiconductor substrate, a lithographic mask may have a pattern with various sizes and shapes. For example, a contact mask may require both square and rectangular openings. Contact masks are employed to form the openings in layers, (typically protective glass layers), to allow contact to underlying devices or metal lines. In the formation of a contact over a device or metal line, more consideration is placed on the aspect of the area being opened rather than the sharp corners or straight lines of the contact opening. The reason for this is because the most important function of a contact is to make reliable contact to the underlying layer or device. Specifically, if a contact is too small, i.e. not a large enough opening to the underlying device, insufficient electrical contact may occur. Conversely, if the contact opening is too large, i.e. covers too much area, shorts may occur due to overlapping of contacts and other electrical lines.
Rectangular shaped contacts offer several advantages to IC fabrication. First, a rectangular contact generally covers a larger area as compared to a square contact having similar dimensions. Since the size of a contact opening is inversely related to its associated contact resistance, a larger rectangular contact will have lower contact resistance than a smaller square contact. Lower contact resistance reduces power consumption and thus overall power consumption of the IC. In addition, a contact having lower resistance is preferable because it is less prone to the effects of electromigration.
Electromigration occurs in metal lines and contacts operating with high current densities at elevated temperatures. In time, electrons in the high current density contacts and lines may cause metal atoms to migrate. If enough metal atoms are displaced, open circuits may occur. Hence, the long-term reliability of an IC may be adversely affected if contacts are too small with respect to their associated current density. So, in the case where high current densities are a concern, it may be desirable to utilize rectangular contact openings in addition to square contact openings.
However, it has been found that problems occur when lithographically transferring square and rectangular shapes on the same mask if the critical dimensions of the shapes approach the resolution limit of the exposure tool. The resolution for an exposure tool is defined as the minimum feature that the exposure tool can repeatedly expose onto the wafer. Currently, the resolution for most advanced optical exposure tools is around 0.5 micron. Also, as stated above, current state-of-the-art IC circuits are being designed with critical dimensions equal to 0.5 microns. Consequently, the problem of printing rectangular and square shapes on the same mask is a prevalent problem.
For example, if the pattern of a square having 0.5 micron sides and a rectangle having a 0.5 micron width is transferred from a mask onto a resist layer, (utilizing an exposure tool having 0.5 micron resolution limit), the center portion of the rectangle transfers relatively wider than, and out tolerance of, the originally designed 0.5 micron rectangle mask width. This is true if the energy setting of the exposure tool is set to ensure that the 0.5 micron square feature transfers to the resist layer within tolerance. The energy setting to ensure that a feature transfers within CD tolerances is referred to as the nominal energy (E.sub.n) for that particular feature.
However, if the energy level of the exposure tool is set to the E.sub.n of the 0.5 micron rectangle, the 0.5 micron rectangle will be transferred within CD tolerances, but the 0.5.times.0.5 micron square shape will be underexposed. In other words, the optimum energy setting to expose a rectangular shape in much less than that of a square shape when the critical dimensions of the shapes approach the resolution limit of the exposure tool.
The reason for the difference in exposure results is that the intensity level measured on the resist for the different shapes is not the same as when the shapes are exposed at the same time utilizing an exposure tool set at a certain energy level. As a result, if a mask pattern containing a rectangle and square are simultaneously transferred to a resist layer, only one of the features is optimally transferred.
An inability to control final resist CDs of contact patterns can result in short circuits due to over exposed rectangular contact openings. Conversely, contact openings may not even be formed due to underexposed square contact openings. Therefore, a method is needed to ensure that the final CDs of all of the shapes within a pattern on a photoresist layer are consistent with the CDs on the original mask regardless of shape.
The most obvious way to avoid this phenomena is to relax the design rules of the circuit such that the CDs are not at the resolution limit of the exposure tool. However, increasing CDs, yields larger die sizes. Larger die and circuit sizes have many undesirable characteristics such as high defect densities and slower circuit speeds. Also, not utilizing the exposure tool to its fullest capabilities is not a cost effective practice.
Pre-biasing mask patterns is another way of overcoming this problem. The original square and rectangle contact mask pattern may be adjusted such that the CDs of the square contacts are increased and the energy of the exposure tool is set at the En of the rectangular shape. This method utilizes the fact that when the dimensions of a square shaped contact is increased, its associated En is decreased. Therefore the size of the square contact can be increased until its En approximately matches the En of the rectangle. The final CDs of both shapes in the photoresist layer are then closer to the original masked CDs. However, the process of trying to increase the square contact size and adjust energy levels is mainly a trial-and-error exercise and is extremely process dependent. Therefore, this method is generally more effective when applied to a somewhat stable processes.
The lithographical mask of the present invention overcomes the problem of transferring square and rectangular shapes by reducing the intensity level on the resist layer of the rectangular shape. As mentioned above, the intensity level measured on a resist layer is much higher in the center of a rectangle than that of the square when exposed to the same radiant source. This results in an overexposure of the center of the rectangle. By altering the mask pattern of the rectangular shape, the intensity level of the center of the rectangle can be modulated thereby eliminating the overexposure problem. The effect of the intensity modulation is to make it possible to transfer both square and rectangle shapes utilizing the same exposure tool and energy level while retaining the same CDs for both shapes.